Many of the biological actions of steroids are mediated by growth factor signaling pathways. Activation of these pathways or related kinases may also be critical for hormonal carcinogenesis and tumor progression in sex steroid target organs, such as the uterus, breast, and prostate gland. In a series of three publications we showed that the ovarian steroid hormone estradiol stimulates formation of a signaling complex in the mouse uterus consisting of the insulin-like growth factor-I receptor (IGF-1R), the insulin receptor substrate-1 (IRS-1), and phosphatidyl inositol-3 kinase (PI-3k). Immunodepletion of IRS-1 in uterine extracts revealed that the PI-3k was associated prdominantly with the IRS-1 docking protein and not directly with the receptor. Based on these data and earlier reports we proposed that this signaling complex formed as a consequence of hormonal stimulation of uterine stromal IGF-1 synthesis and subsequent activation of epithelial IGF-1R by this ligand. Three other related research questions were considered. (A) We sought to determine whether IGF-1R is critical for estrogen-stimulated mitosis in the uterine epithelium (B) We also examined selected clinical and environmental estrogens to determine whether they stimulate formation of the IGF-1/IRS-1 complex and also were mitogenic in the mouse uterine model. Transgenic mice deficient in the estrogen receptor (ER)-a were used in this study to ascertain the importance of this receptor for IGF-1R activation by various estrogens. (C) While the steady-state levels of IRS-1 remain fairly constant in the uterus in response to estradiol, a related docking protein, IRS-2, with a similar modular structure transiently disappears. A study is described to determine the relevant mechanisms that cause this down regulation and also to understand what signaling function(s) is served by the IRS-2 docking protein. The development of the mammary gland occurs in distinct stages that are manifested in the structure and function of cells in this organ. Multiple systemic hormones and local regulatory factors have been previously shown to be essential for both the morphogenesis and differentiation of the mammary gland. We have focused on the roles of members of the erbB family and related ligands because approximately one-third of breast carcinomas exhibit amplification and/or overexpression of the erbB-2 receptor. In addition, we previously published that the EGF receptor (EGFR or erbB-1) is essential for mammary ductal morphogenesis. This conclusion was based on a combination of different methods that included in vivo evaluation of tyrosine phosphorylation of members of the erbB family and assessment of morphogenesis in EGFr-/- mammary glands. This investigation also established that EGFR functions during this period of mammary development as a heterodimer with erbB-2 in the mammary stroma. In a study described briefly below we report that a mammary erbB-2/erbB-3 heterodimer may function during late pregnancy and lactation and also in mammary hyperplasias and tumors of Wnt-1 transgenic mice. Several recent publications have stimulated interest in IGF-1 and breast cancer risk based on the fact that elevated serum levels of this growth factor are associated with an increased incidence of this disease. Mammographic density, which is positively associated with breast cancer risk, correlate with serum IGF-1 levels. This prompted us to study mammary gland development in mice with mutations affecting IGF-1 synthesis or IGF-1R signaling.